In over-the-air communications systems (“wireless systems”), cost of bandwidth may be a significant factor in overall system cost. Accordingly, accommodating many users in a system may be a means for enhancing bandwidth usage to reduce cost to individual users.
To accommodate many users in a system, multiband communication systems conventionally include a transmitter that provides a transmission signal which is a combination of baseband signals. Conventionally, these baseband signals have independent statistical characteristics occupying non-overlapping frequency bands (“sub-bands”). Because these sub-band signals are combined in a transmission signal, there is a potential for super-positioning of signals where peaks of a combined signal are sufficiently additive, or subtractive in the negative direction, to exceed a peak threshold.
Accordingly, signal peak-to-average power ratio (“PAPR”) is a transmitter characteristic. Because PAPR of a superposed multiband signal may be substantially high, to accommodate such peaks would lead to high power consumption in a radio frequency amplifier or other power amplifier. Moreover, to provide a digital-to-analog converter (“DAC”) for example that would accommodate such possible peaks would be more costly than a DAC that would not accommodate such peak dispersion. Moreover, nonlinear characteristics of a power amplifier conventionally increase exponentially with the increase of PAPR of a signal.
Thus, it should be apparent that bandwidth efficiency and transmission power efficiency may be conflicting criteria in design of a digital transmission system, or more particularly, design of a digital front-end architecture for a multiband transmitter.
One way of mitigating PAPR is to clip peaks of a signal prior to digital-to-analog conversion and power amplification. However, when one clips or otherwise removes a portion of the signal, there may be a consequence of reducing transmitter performance. One measure of this reduction is quantified in terms of error vector magnitude (“EVM”). EVM is the ratio of transmitter induced noise power to the undistorted transmit signal of such transmitter.
Moreover, clipping a signal in a multiband application may lead to generating noise not just in one sub-band but in one or more other sub-bands. A quantification of this introduced noise from one band to another is known as adjacent channel leakage ratio (“ACLR”).
It should be understood that a transmitter design approach that facilitates finding an intersection of PAPR, EVM, and ACLR solution spaces would be desirable and useful for transmitter design. Heretofore, others have proposed crest factor reduction (“CFR”) algorithms for a particular modulation format, such as multi-carrier wide-band code division multiple access (“WCDMA”) and orthogonal frequency division multiplexing (“OFDM”), among other known modulation formats. Moreover, others have proposed CFR for single-band transmitters.
Accordingly, it would be desirable and useful to provide a transmitter design which is less dependent on modulation format than previously thought. More particularly, it would be desirable and useful to provide a transmitter with CFR configured to accommodate any of a variety of multiband transmitters.